Soft Selection Rules for Femtosecond Pump-Probe Vibrational Coherence Spectroscopy

Persevering research efforts that seek to harness the potential of singlet fission to increase the efficiency of photovoltaic devices have recently focused on vibronic mechanisms that presumably underlie this process. The fundamental theoretical input needed for treating the transitions between the electronic states involved are the Franck-Condon parameters that govern the generation of the coherent vibrational wave packets emerging in the fission phenomenon and in the experiments performed for its investigation. Their calculation for triplet states is not commonly practiced, so they are usually simulated by their singlet counterparts. On the basis of the analysis of coherence flow descriptors, here we find the validity of this approximation questionable for subtle interpretative issues of femtosecond pump-probe vibrational spectroscopy where a slight difference in a mode's coherence power may have a pivotal role in assessing the mechanism of fission dynamics. In less sensitive instances, the approach is acceptably reliable at qualitative level, apart from a tendency to predict sporadic spurious coherences. Our methodology may lead to a potential tool for detecting those artifacts.